A radio-frequency (rf) atmospheric-pressure discharge in He–O2 mixture is studied using a
fluid model for its wall fluxes and their dependence on electron and chemical kinetics in the
sheath region. It is shown that ground-state O, O+2 and O− are the dominant wall fluxes of
neutral species, cations and anions, respectively. Detailed analysis of particle transport shows
that wall fluxes are supplied from a boundary layer of 3–300μm immediately next to an
electrode, a fraction of the thickness of the sheath region. The width of the boundary layer
mirrors the effective excursion distance during lifetime of plasma species, and is a result of
much reduced length scale of particle transport at elevated gas pressures. As a result, plasma
species supplying their wall fluxes are produced locally within the boundary layer and the
chemical composition of the overall wall flux depends critically on spatio-temporal
characteristics of electron temperature and density within the sheath. Wall fluxes of cations
and ions are found to consist of a train of nanosecond pulses, whereas wall fluxes of neutral
species are largely time-invariant.

This work was supported by the State Key Laboratory of
Electrical Insulation and Power Equipment (No EIPE11108),
the Fundamental Research Funds for the Central Universities
of China, and the English Department of Health.